One technique used to obtain a security paper for the printing of security documents and/or banknotes consists in inserting a security thread or strip during the paper manufacturing process. It is well known in the manufacture of security paper that the most common technique for shaping the sheet of paper is the so-called round technique, consisting in a metal drum that filters water that has cellulose fibres in suspension that thus remain on the surface of the drum whilst the water penetrates inside the drum. Similarly, it is also known that for the thread to remain perfectly integrated into the paper, it must be inserted during this stage of formation of the paper sheet, and consists in incorporating the thread around the drum onto which the cellulose fibres are being deposited, thus remaining embedded inside the fibres that are deposited on the surface of the drum.
As the water crosses the drum mesh, the fibres weave themselves together and the security thread or strip is enveloped by the mesh formed by said fibres as the sheet of paper is formed.
A second layer of paper created at the same time on another drum can be added to this first layer so that they remain intimately linked together, having been joined during their formation, and which together will make up the security paper.
A sheet of paper manufactured in this way, with the security thread or strip inside it, will then go through the subsequent stages of the manufacturing procedure in order to produce the security paper.
Although this is the most common way to manufacture banknote paper with embedded security threads, we shall also highlight other paper manufacturing processes based on flat top technology instead of the metal mould process.
There are patents dealing with security strips of relevant features, amongst which we find European Patent EP-0319157, relating to security threads incorporating drawings or legends performed on a metal layer where the partial demetallisation of such layer allows obtaining such security drawings or legends that would be visible to the naked eye.
British Patent GB-2,260,772 shows how to manufacture a document incorporating an element called windowed security threads where the thread embedded inside the security paper is exposed at spaced locations, thus being visible on the security document, which could be a banknote, as sequence of windows through which the security thread can be seen and remaining hidden in the rest of the document.
This facilitates locating such a security strip on the document and therefore facilitates the verification of the authenticity of the document as well as including the possibility of incorporating graphic designs on the thread that will highlight its presence in the document and improve its identification by a user.
This windowed thread effect is obtained using the metal mould paper manufacturing method by providing a series of raised portions on the drum surface on which the security thread rests.
When the security strip comes in contact with the raised portions of the formation drum, this space is not filled with fibres, such that the security paper or security document will have windows through which the security thread or strip will remain visible. This product is usually called windowed thread, since the security thread or strip is shown through the windows performed on the security paper.
Another security measure that can be incorporated to security documents are the so-called watermarks, which effect is achieved by more or less opacity in the document in different areas according to more or less fibres having been deposited during paper manufacture.
This depositing of more or less fibres is achieved by means of raised portions and recesses forming images or fanciful drawings on the mould used to manufacture the paper. The raised portions will produce a thinner layer of fibres on the paper manufactured, thus giving rise to lighter or less opaque areas, and the recesses on the drum will produce a greater accumulation of fivers leading to the formation of opaque areas that will not allow the passage of light. This combination of light and dark areas on the paper is what is called a watermark.
One type of watermark that is also known is described in European Patent EP-0549384, which is the technique known as the high contrast single-tone watermark. This technique allows inserting several elements, called electrotypes, in the paper manufacturing mould. These electrotypes prevent drainage in the areas of the paper they are placed in, that is, they prevent drainage of the water accompanying the fibres when they are deposited on the surface of the paper manufacturing mould. By preventing drainage, in the areas where these elements are inserted there is less deposition of fibres and therefore areas with less opacity.
These less opaque areas can be made more or less obvious depending on the relative height of said insertions.
Each one of these techniques, the windowed thread and the high contrast or electrotype watermark, have been used separately within a security paper or security document in order to offer a certain degree of safety to the document that includes them.